As costs of energy increase along with concerns about global warming due to consumption of fossil fuels to generate energy, there is an every increasing need for more efficient lighting technologies. These demands, coupled with rapid improvements in semiconductors and related manufacturing technologies, are driving a trend in the lighting industry toward the use of light emitting diodes (LEDs) or other solid state light sources to produce light for general lighting applications, as replacements for incandescent lighting and eventually as replacements for other older less efficient light sources.
The actual solid state light sources, however, produce light of specific limited spectral characteristics. To obtain white light of a desired characteristic and/or other desirable light colors, lighting devices based on solid state sources have typically used sources that produce light of two or more different colors or wavelengths. One technique involves mixing or combining individual light from LEDs of three or more different wavelengths (single or “primary” colors), for example from Red, Green and Blue LEDs. Another approach combines a white LED source, which tends to produce a cool bluish light, with one or more LEDs of specific wavelength(s) such as red and/or yellow chosen to shift a combined light output to a more desirable color temperature. Adjustment of the LED outputs offers control of intensity as well as the overall color output, e.g. color and/or color temperature of white light.
To provide efficient mixing of the various colors of the light and a pleasing uniform light output, Advanced Optical Technologies, LLC (AOT) of Herndon, Va. has developed a variety of light fixture configurations that utilize a diffusely reflective optical integrating cavity to process and combine the light from a number of solid state sources. By way of example, a variety of structures for AOT's lighting systems using optical integrating cavities are described in US Patent Application Publications 2007/0138978, 2007/0051883 and 2007/0045524, the disclosures of which are incorporated herein entirely by reference.
Although these integrating cavity based lighting systems/fixtures provide excellent quality light in an efficient manner and address a variety of concerns regarding other solid state lighting equipment, there is still room for improvement. For example, efficiency of the optical integrating cavity decreases if the diffuse reflectivity of its interior surface(s) is compromised, for example due to contamination from dirt or debris entering the cavity. Also, since the cavity is filled with air (low index of refraction), some light may be trapped in the LED packages by internal reflection at the package surface because the material used to encapsulate the LED chip may have a higher index of refraction. Efficiency may also be somewhat reduced if the mask or portion of the cavity around the aperture needs to have a relatively large size (producing a small optical aperture) to sufficiently reduce or prevent direct emissions from the solid state light source(s) through the cavity and optical aperture.
Hence a need exists for techniques to further improve optical integrating cavity type solid state lighting fixtures or systems.